The present invention relates to an optical gas temperature sensor, and more particularly to a high temperature probe therefor.
Optical gas temperature sensors are well known in the art. Such a sensor is conventionally used by immersing the probe (and more particularly the conical tip of the probe) in the hot gas stream to be measured, and allowing the probe to remain until it is heated to approximately the temperature of the hot gas stream. At that temperature, infrared light (typically 1.0 to 1.7 microns in wave length) is emitted from the inner surface of the probe tip (which may contain a separate emitter), selected by an optical lens within the tip, and focussed by the lens onto a fiber optic or light guide for transmission to a photodiode. The photodiode converts the focussed light into an electrical current, amplifiers in an opto-electronic unit condition the analog signal, and a program in a computer or microprocessor converts the analog signal into usable engineering units of temperature.
U.S. Pat. No. 4,770,544 describes an optical gas temperature sensor having a high temperature probe formed of a single crystal sapphire rod divided into a waveguide region and a cavity region. The cavity region is generally conical and is coated with an infrared radiation emitter having a high melting temperature, such as sputtered iridium. The iridium coating is in turn being covered by a protective coating, such as sputtered alumina. The nature of the materials used to form the high temperature probe of an optical gas temperature sensor may vary considerably. Some materials require the use of a separate emitter in order to provide infrared radiation in response to the sensed temperatures, while others do not. Many of the probes utilize materials such as refractory materials (including oxides of aluminum, silicon, zirconium and yttrium), black bodies formed of finely dispersed carbon and a silicon adhesive, quartz or glass, noble metals, steel, luminescent materials, and the like.
While a variety of different materials have been used for the probes, as noted above, the most common probes are sapphire probes provided in various shapes and with various coatings. However the sapphire probes have not proven to be entirely satisfactory. The probe tips are subjected to extreme thermal shock (on the order of 1,000.degree. F. per second), high temperature stress, oxidizing and salt atmospheres at high temperatures (e.g., those found in aircraft engines), and the like. The sapphire probes tend to fracture quickly when cooled from 2500.degree. F. to 70.degree. F. by air nozzles in tests that approximate an engine environment. Sapphire probes under a 5000 psi bending stress can creep or deform at 2500.degree. F. and are not well suited for meeting 5000-hour life requirements. Where the emissivity of the sapphire is supplemented by an emitter coating (such as iridium) or fitted with an insert of emitting material, the coating or emitted insert tends to erode within several hours of exposure to engine gas, with substantial temperature errors resulting. Accordingly, the need remains for a high temperature probe for an optical gas temperature sensor having a high flexural strength (defined as exceeding 50,000 psi at 2,500.degree. F. on a four-point bending test), a low creep rate (defined as a creep rate of 5.times.10.sup.-10 sec.sup.-1 with up to 5,000 psi stress at 2,500.degree. F. which produces 1% strain over a 5,000 hour life), a high oxidation resistance (defined as less than 1% weight loss for a 5,000-hour life at 2,500.degree. F. and as assessed during a 3,500-hour thermal cycling test using exhaust gases), and a high thermal shock resistance (defined as the capacity to withstand repeated cycling shocks from 2,500.degree. F. to 1,000.degree. F. within 3 seconds or 500.degree. F./second, as could be applied with a high velocity torch and air gun). Additionally, the probe must exhibit a low thermal response time constant so that it responds rapidly to variations in the temperature of the gas stream. A one-second time constant under engine air flow conditions is generally acceptable and is the current practice with conventional thermocouple and engine control designs. Thin walls are required at the probe tip to insure adequately fast thermal response.
U.S. patent application Ser. No. 916,791, filed Jul. 17, 1992, now U.S. Pat. No. 5,277,496, discloses such a non-sapphire high temperature probe for an optical gas temperature sensor. Nonetheless, even this probe is susceptible to improvement. As earlier noted, an optical lens is supported in the tip of the probe and is responsive to the temperature of the gas stream proximate the tip for focusing and transmitting electromagnetic radiation emitted from the tip (in response to the temperature of the gas stream proximate the tip) in order to effect the measurement of the gas stream. However, as is well known in the optical probe art, the maintenance of the probe tip at a high temperature results in the high temperature emissions of impurities which accumulate on the optical lens and eventually interfere with its efficiency. In probes of the type described, the optical gas was a fixed and permanent part of the probe tip and could not be removed therefrom for repair (that is, cleaning to remove the impurities collected thereon from the high temperature emissions of the tip) or, when necessary, replacement thereof. Thus, although the optical lens is but a small factor in the total cost of the probe, the optical lens could not be replaced or repaired without replacement of the much more costly probe itself.
Further, while the material of the joint was selected to accommodate the thermal stresses and the mechanical stresses to which the probe was subjected, there remained a high failure rate for the tip where it was contacted by the joint or the high temperature braze between the tip and the joint.
Accordingly, it is an object of the present invention to provide an improved high temperature optical probe for an optical gas temperature sensor.
Another object is to provide such a probe which has a high flexural strength, a low creep rate, a high oxidation resistance, and a high thermal shock resistance.
A further object is to provide such a probe which has a low thermal response time constant.
It is also an object of the present invention to provide such a probe wherein the optical means is easily removable from the tip for repair (including cleaning) and/or replacement.
It is another object to provide such a probe wherein in one embodiment the joint has a geometry which minimizes failure of the tip at and adjacent the joint/tip interface.
It is a final object to provide such a probe which is of simple and economical construction, easy to maintain, and easy to use.